1. Field of the Invention
The invention relates to a high temperature fuel cell with a thin film electrolyte in accordance with the preamble of claim 1. Such a fuel cell is known from Publication EP-A 0 722 193. The invention also refers to a battery with fuel cells arranged in stacks.
2. Description of the Prior Art
In the known fuel cells, the electrochemically active element, which consists of an anode, a solid electrolyte and a cathode in a planar arrangement, is manufactured by means of a vacuum-plasma injection process, a VPS process for short.
Thin film electrolytes, such as can be manufactured by the VPS process, permit operating temperatures of 600 to 800.degree. C. In electrochemically active elements in the form of self supporting plates, as are customary in the older types of fuel cells, the operating temperatures lie between 900 and 1000.degree. C. Various problems in connection with the choice of material which give rise to high costs at the higher temperatures do not arise at the lower temperatures. Nameworthy cost reductions in comparison with the older types of fuel cells are therefore to be expected in the case of fuel cells with thin film electrolytes. The long term stability of the fuel cells is also improved.
In the known fuel cell, the carrier structure for the electrochemically active element is a flexible porous body built up of layers. In a boundary zone of this carrier structure, the pores, which are relatively large, are filled with fine metallic particles. This boundary zone represents a fine pored layer onto which the thin film layers of the electrochemically active element can be applied. The flexibility of the carrier structure has the advantageous effect that damaging thermal strains between the ceramic electrolyte and the metallic carrier structure can be avoided. The proposed use of a metallic felt for the carrier structure is however problematic with respect to the manufacture of the electrochemically active element by means of a thin film technique, since it is difficult to manufacture a substrate suitable for application of the thin film layers by means of the disclosed process. A substrate is suitable if it has a smooth, homogeneous surface free of defects in spite of its porosity.